Natural gas is obtained from underground reservoirs and pumped through pipelines to various industrial and commercial consumers. Much of the natural gas is utilized for heating purposes and, accordingly, requires a BTU content of only 900 to 1000 BTU per m.c.f. A natural gas stream composed mainly of methane and ethane is sufficient to achieve such heating values. However, much of the natural gas obtained from underground reservoirs is rich in other components, such as ethane, propane, pentane and butane, which are heavier than methane and ethane. These components are industrially valuable in many processes, and, accordingly, separation of them and ethane from the methane prior to burning of the natural gas is highly desirable. Separation is usually accomplished at cryogenic temperature with distillation to separate and return methane to the gas pipeline while retaining a significant percentage of the ethane and heavier components.
Many processes have been devised for the cryogenic separation of methane from heavier components in a natural gas stream and for cryogenic refrigeration. Among these are U.S. Pat. Nos. 4,072,485 to Becdelievre et al; 4,022,597 to Bacon; 3,929,438 to Harper; 3,808,826 to Harper et al; Re. 29,914 to Perret; Re. 30,085 to Perret; 3,418,819 to Grunberg et al; 3,763,658 to Gaumer, Jr. et al; 3,581,510 to Hughes; 4,140,504 to Campbell et al; 4,157,904 to Campbell et al; 4,171,964 to Campbell et al; 4,278,457 to Campbell et al; 3,932,154 to Coers et al; 3,914,949 to Maher et al; and 4,033,735 to Swenson.
Several of these patents, such as those to Campbell et al and Bacon, utilize a cooling mechanism by turbo expansion. Several use a plurality of successive, staged, external, indirect heat exchangers and/or totally condensing refrigerant, such as Becdelievre, Harper, Harper et al, Gaumer et al, Hughes, Coers et al, and Grunberg et al. Others use multistage flash systems separating refrigerant at various levels of temperature and pressure which emulates a cascade system in a closed loop mixed refrigerant scheme, such as those to Perret. Several also use single or very limited component refrigerant composition, such as Maher et al. Others use various schemes for refrigeration such as Swenson.
None of this prior art shows the process of the present invention for producing the cryogenic temperatures required for separation.